Undeveloped watershed

The impacts to stream channels are different at different stages of urbanization. Undeveloped watersheds typically exhibit well-functioning natural systems, with little pollution and well-vegetated stream channels.

Commencement of development

When rural or undeveloped areas are just beginning to become developed, with the addition of some houses and the removal of some of the trees and vegetation, impacts include an increase in storm runoff and erosion. This is because there are fewer plants to slow the overland flow. As a result, more sediment is washed into streams.

Beginning large-scale urbanization

As more land is cleared for the construction of houses and subdivisions, sediment is washed into stream channels. Streams may be diverted to provide a water supply for the increasing population. Greater numbers of buildings and parking lots increases the amount of impervious surface, lessening infiltration and increasing runoff. The increase in pollutants reaching the stream can have detrimental impacts on fish and aquatic organisms in the stream.

Continued large-scale urbanization

The continued addition of houses, roads, parking lots, and commercial and industrial buildings represents continued urbanization. This further increase in impervious surface within the watershed continues to negatively impact infiltration and lead to runoff and the introduction of pollutants into the stream channel. Under these conditions, precipitation falling within the watershed can rapidly reach the stream channel, greatly increasing the channel flow for a period of time. This increased flow can lead to channel scour and downcutting. The increase in built-up areas within the watershed can also lead to an increase in pollutants and wastewater reaching the stream, adversely affecting stream life.